在Validator中使用EntityManager

Question

我实现了自己的验证器，它实现了接口javax.faces.Validator。对于我的验证，我需要一个EntityManager来创建一个查询。这是我的代码：

@FacesValidator(value = "userAliasValidator")
public class UserAliasValidator implements Validator {

    @PersistenceUnit
    private EntityManagerFactory emf;
    private EntityManager em;

    @Override
    public void validate(FacesContext context, UIComponent component,
            Object object) throws ValidatorException {
        em = emf.createEntityManager();
        Query query = em.createNamedQuery("SelectUserByAlias");
        String alias = String.valueOf(object);
        query.setParameter("alias", alias);

        List<User> result = query.getResultList();

        if (result.size() > 0) {
            FacesContext.getCurrentInstance()
                    .addMessage(
                            null,
                            new FacesMessage(FacesMessage.SEVERITY_ERROR,
                                    "User allready exists",
                                    "Please try another name!"));
        }
    }
}

不幸的是，这不起作用。如何在验证器中使用EntityManager？